Her account of his medical journey illustrates just how difficult it is for a typical Lewy body disease patient to get properly diagnosed, how prescribed medicines for misdiagnosed conditions may have exacerbated his symptoms, and how patients who are properly diagnosed have no cure for their disease.

“I am not convinced that the knowledge would have done much more than prolong Robin’s agony,” she wrote. “Even if we experienced some level of comfort in knowing the name, and fleeting hope from temporary comfort with medications, the terrorist was still going to kill him. There is no cure and Robin’s steep and rapid decline was assured.”

Physical symptoms lead to cognitive problems

Susan begins her letter describing the fall of 2013. By then, Robin was already seeing a doctor to cope with physical symptoms like a tremor in his left hand, constipation, heartburn, insomnia and a poor sense of smell. Then, one weekend in late October, she observed that his fear and anxiety had “skyrocketed” to alarming levels, beyond his usual stress.

By winter of that year, Robin was having problems with paranoia, delusions, insomnia, memory and high levels of the stress hormone cortisol. He sought psychotherapy to help him cope with his fear and anxiety.

In April 2014, he had a panic attack while filming “Night at the Museum 3.” During filming, he struggled to memorize his lines ― unusual for the Juilliard-trained actor ― and his doctor prescribed antipsychotic medications.

Susan would later find out, months after her husband’s death, that antipsychotics can cause severe reactions in people with LBD, and in some cases even worsen their cognitive and physical symptoms.

Finally in May, doctors diagnosed Robin Williams with Parkinson’s disease, which would explain physical symptoms like his hand tremor and difficulty moving. While the diagnosis proved comforting for a while, Susan writes that Robin sensed something was still deeply wrong ― specifically with his brain and his cognitive behavior.

“I just want to reboot my brain,” he said on one occasion. “Do I have Alzheimer’s? Dementia? Am I schizophrenic?” he asked on another.

A brief respite from the storm of symptoms

For the rest of the summer, the couple focused on dealing with his symptoms. He went to therapy and worked out with his trainer, and he learned meditation, yoga, self-hypnosis and more to calm his anxiety.

By July they decided to do more tests to get to the root of his depression and anxiety, and he switched medication. In August, it seemed the change had worked and his symptoms were improving ― but on Aug. 11, 2014, an assistant found his body in his home. Robin Williams had killed himself.

Three months later, his widow learned from his autopsy report that his brain had been riddled with Lewy bodies, abnormal protein deposits that develop throughout the brain.

“The massive proliferation of Lewy bodies throughout his brain had done so much damage to neurons and neurotransmitters that in effect, you could say he had chemical warfare in his brain,” she wrote.

How is Lewy body disease diagnosed?

While the presence of Lewy bodies is one of many symptoms that both Parkinson’s disease and Lewy body disease share, dementia with Lewy bodies is different in that cognitive symptoms quickly follow movement problems. In Parkinson’s disease, the cognitive symptoms appear more than a year after the physical symptoms first start. However, researchers acknowledge this cut-off is somewhat arbitrary, and that they’re both diseases on the same spectrum.

In both cases, there is also no test that can definitively confirm the disease except a final autopsy; doctors have to make a clinical diagnosis based on a constellation of confusing symptoms. There is also no cure, but there are therapies that can perhaps prolong the milder stages of the disease and help patients and caregivers cope with the changes.

Dementia with Lewy bodies has a few defining traits: progressive dementia, features of Parkinsonism, disordered sleep and hallucinations. While Robin Williams never admitted to having hallucinations, a doctor reviewed his medical records after his death and said it was likely he did have hallucinations but was keeping this information to himself.

Williams’ widow ended her letter with a note of encouragement for neurologists to continue their work researching cures for brain diseases. She also thanked them for their future work, and said she hopes some might turn out to discover the cure for LBD.

Pat Snyder, a woman whose husband died of Lewy body dementia in 2015, told HuffPost in a previous story that the couple visited 12 doctors over seven years before she found someone who could properly treat him.

“Her story is our story,” Snyder had said about Susan Schneider Williams last year. “You’re playing whack-a-mole, getting symptoms you don’t understand [and] you’re going to doctors who don’t understand, don’t diagnose it. I am thrilled that she has been willing to step out and tell the story and make America more aware of this, because it’s very common.”